Passivating layer for photovoltaic cells

ABSTRACT

A photovoltaic cell which comprises a first electrode, a second electrode, a photoactive, charge-separating layer comprising a semiconducting polymer between the first and the second electrodes, and a passivatng layer adapted to enhance the lifetime of the photovoltaic cell. The passivating layer comprises a substantially amorphous titanium oxide having the formula of TiO x  where x represents a number from 1 to 1.96.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/347,111 filed Feb. 2, 2006, which claims the benefit ofpriority under 35 USC § 119(e) to U.S. Provisional Patent ApplicationSer. No. 60/663,398 filed Mar. 17, 2005, and which is acontinuation-in-part of U.S. patent application Ser. No. 11/326,130filed Jan. 4, 2006, which in turn claims the benefit of priority under35 USC § 119(e) to U.S. Provisional Patent Application Ser. No.60/663,398 filed Mar. 17, 2005, the disclosures of all of which areincorporated herein by reference in their entirety. This Applicationclaims the benefit of priority under 35 USC § 119(e) to U.S. ProvisionalApplication Ser. Nos. 60/756,604 filed Jan. 4, 2006, and 60/872,401filed Feb. 1, 2006, the disclosures of which are incorporated herein byreference in their entirety.

BACKGROUND

This invention relates generally to polymer-based electronic devices andin particular to photovoltaic cells comprising titanium oxides withimproved device efficiency, performance and lifetime.

Electronic devices based on semiconducting and metallic polymers providespecial opportunities for novel products as they can be fabricated inlarge areas using low cost printing and coating technologies to depositand simultaneously pattern active electronic materials on lightweightflexible substrates. Products based on printed plastic electronics areexpected to develop into a significant industry with a more than $100billion market opportunity that is enabled by a new generation oflow-cost, lightweight, and flexible electronic devices.

Although electronic devices such as diodes, field effect transistors(FETs), light-emitting diodes (LEDs), solar cells, and photodetectorsfabricated from semiconducting and metallic polymers have beendemonstrated with performance comparable to or in some cases even betterthan their inorganic counterparts, the typically short lifetime of thepolymer-based devices must be overcome before large scalecommercialization can be realized. Most conventional semiconductingpolymer materials are degraded when exposed to water vapor and/or oxygenin the air. Photo-oxidation is often a serious problem to polymer-basedelectronic devices.

The degradation of polymer devices can be eliminated or at least reducedto acceptable levels by sealing the components inside an impermeablepackage using glass and/or metal (sometimes with a desiccant inside) toprevent exposure to oxygen and water vapor. Attempts to create flexiblepackaging using hybrid multilayer barriers comprised of inorganic oxidelayers separated by polymer layers with total thickness of 5-7 μm havebeen reported with promising results. Although such encapsulationmethods can reduce oxygen and moisture permeation, they are expensiveand typically result in increased thickness and loss of flexibility. Toachieve the goal of simple fabrication by solutionprocessing—flexibility and thin film factor for printed plasticelectronics—improved barrier materials for packaging and/or devices withreduced sensitivity are needed to enable large scale commercializationon plastic substrates.

Photocatalysis by titania (TiO₂) has been extensively investigated,especially for air and water purifications. These applications are basedon photogeneration of electron-hole pairs by absorption of photons withenergies greater than the band gap (in the ultraviolet) ofnanoparticulate TiO₂ suspensions or films. These relatively high energyelectron-hole pairs can react at the TiO₂ surface to drivephotocatalytic or photosynthetic redox reactions. If appropriateelectron acceptors (e.g., oxygen) and electron donors (e.g., organicmolecules) are adsorbed onto a semiconductor surface, interfacialelectron-transfer reactions take place, resulting in, for example,complete photo-mineralization of the organic to carbon dioxide, water,and mineral acids. During the process, oxygen consumption is a principalfactor in the photocatalytic reaction. In addition, because Ti issufficiently reactive the oxygen-deficient surfaces are expected toreact with O₂. Studies have shown that TiO₂ has a substantial oxygenscavenging effect originating from the combination of the photocatalysisprocess and oxygen deficiencies within the structure. As a consequence,TiO₂ has been developed as an active packaging material foroxygen-sensitive products such as pharmaceuticals, medical instruments,museum pieces, and oxygen-sensitive foods.

For many reasons water is also an important adsorbate on TiO₂ surfaces.Many applications and in fact most photocatalytic processes areperformed in the presence of water vapor. Ambient water vapor interactswith TiO₂ surfaces, and the resulting surface hydroxyls can affect theadsorption and reaction processes. The adsorption of water on TiO₂ hasbeen of intense interest in recent years.

The various aspects of the photocatalytic activity of TiO₂ are reviewedextensively in the art. The main features of the process can be brieflysummarized as follows. The primary excitation results in an electron inthe conduction band and a hole in the valence band. When TiO₂ is incontact with an electrolyte, the Fermi level equilibrates with the redoxpotential of the redox couple. The resulting Schottky barrier drives theelectron and the hole in different directions. The components of theelectron-hole pair, when transferred across the interface, are capableof reducing and oxidizing an adsorbate, forming a singly oxidizedelectron donor and a singly reduced electron acceptor, as shown indetail in the following equations:TiO₂+hv→TiO₂(e⁻, h⁺)  (1)TiO₂(h⁺)+RX_(ads)→TiO₂+RX_(ads) ^(•+)  (2)TiO₂(h⁺)+H₂O_(ads)→TiO₂+OH_(ads) ^(•)+H⁺  (3)TiO₂(h⁺)+OH_(ads) ⁻→TiO₂+OH_(ads) ^(•)  (4)TiO₂(e⁻)+O_(2,ads)→TiO₂+O₂ ^(•−)  (5)TiO₂(e⁻)+H₂O_(2,ads)→TiO₂+OH⁻+OH_(ads) ^(•)  (6)

These processes generate anion or cation radicals which can undergosubsequent reactions. Hydroxyl radicals are generally considered themost important species in the photocatalytic degradation of organics,although not in UHV-based studies. It is generally believed that holecapture is directly through OH and not via water first, i.e. through Eq.(4) rather than Eq. (3). The 1b₁ orbital of water lies above the 1πlevel of OH, so one might expect water to be better at capturing a holethan OH, but the radical-cation of water may be neutralized beforedecomposing into an OH radical. Also, it is mostly assumed that thesurface is OH covered and therefore the hole is directly transferred toOH.

The photocatalytic activity of TiO₂ is completely suppressed in theabsence of an electron scavenger such as molecular oxygen. Because theconduction band of TiO₂ is almost isoenergetic with the reductionpotential of oxygen in inert solvents, adsorbed oxygen serves as anefficient trap for photogenerated electrons. The resulting species,superoxide, O₂ ^(•−), is highly reactive and can attack other adsorbedmolecules. Several other oxidation processes, in addition to reactionsshown in Eq.(1)-(6) can occur as well. Often, loading of TiO₂ with Ptand addition of H₂O₂ [Eq.(6)] enhance the overall efficiency of thephotocatalytic degradation processes.

In order for photocatalysis to be efficient, electron-hole pairrecombination must be suppressed before the trapping reactions occur atthe interface. The recombination reaction occurs very fast, and theresulting low quantum efficiency is one of the main impediments for theuse of TiO₂. Degradation of airborne pollutants has resulted in anexplosion of TiO₂-permeated paints and papers to clean up everythingfrom cigarette smoke to acetaldehyde.

TiO₂ has substantial oxygen/water scavenging effects originating fromthe combination of photocatalysis and inherent oxygen deficiency of theTiO₂ structure. Since oxygen and water vapor are principally responsiblefor degradation of polymer devices, incorporation of TiO₂ into or ontopolymer devices seems to be an ideal solution for reducing thesensitivity of such devices to oxygen and water vapor.

However, since crystalline TiO₂ layers (anatase or rutile phase) canonly be prepared at temperatures above 450° C., the formation of aprotective TiO₂ layer in/on the device structure is not consistent withthe fabrication of polymer electronic devices. Active organic layerscannot survive such high temperatures.

The following documents include information generally related to thisinvention and are incorporated herein by reference in their entirety.

-   1. G. P. Collins, Scientific American, August 2004, p. 76,    2004; W. E. Howard, Scientific American, February 2004, p. 76    (2004).-   2. H. Tomozawa, D. Braun, S. Pillips, A. J. Heeger, and H. Kroemer,    Synth. Met. 22, p. 63, (1987).-   3. H. Sirringhaus, N. Tessler, and R. H. Friend, Science 280, p1741    (1998).-   4. J. H. Burroughes, D. D. C. Bradley, A. R. Brawn, R. N.    Marks, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 335, p.    539 (1990).-   5. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science    270, p. 1789 (1995).-   6. G. Yu and A. J. Heeger, J. Appl. Phys. 78, p. 4510 (1995).-   7. R. D. Scurlock, B. Wang, P. R. Ogilby, J. R. Sheats, and R. L.    Clough, J. Am. Chem. Soc. 117, p. 10194 (1995)-   8. K. Z. Xing, N. Johansson, G. Beamson, D. T. Clark, J-L. Bredas,    and W. R. Salaneck, Adv. Mater. 9, p. 1027 (1997).-   9. P. E. Burrows, V. Bulimic, S. R. Forrest. L. S. Capuche, D. M.    McCarty, and M. E. Thompson, Appl. Phys. Let. 65, p. 2922 (1994).-   10. M. S. Weaver, L. A. Michaels, K. Raja, M. A. Rothman, J. A.    Silver nail, J. J. Brown, P. E. Burrows, G. L. Graff, M. E.    Gross, P. M. Martin, M. Hall, E. Mast, C. Bonham, W. Bennett, and M.    Turnoff, Appl. Phys. Let. 81, p. 2929 (2002).-   11. B. Chwang, M. A. Rothman, S. Y. Mao, R. H. Hewitt, M. S.    Weaver, J. A. Silvernail, K. Rajan, M. Hack, J. J. Brown, X. Chu, L.    Moro, T. Krajewski, N. Rutherford, Appl. Phys. Lett. 83, p. 413    (2003).-   12. Fujishima and K. Honda, Nature 238, p. 37 (1972).-   13. R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A.    Kitamura, M. Shimohigoshi, and T. Watanabe, Nature 388, p. 431    (1997).-   14. U. Diebold, Surface Science Reports 48, p. 53 (2003).-   15. Mills, H. R. Davies, and D. Worsley, Chem. Soc. Rev. 22, p. 417    (1993).-   16. O. Legrini, E. Oliveros and A. M. Braun, Chem. Rev. 93, p. 671    (1993).-   17. Heller, Acc. Chem. Res. 28, p. 503 (1995)-   18. M. Hoffman, S. Martin, W. Choi, and D. Bahnemann, Chem. Rev.    95, p. 69 (1995).-   19. L. Linsebigler, G. Lu, and J. T. Yates Jr., Chem. Rev. 95, p.    735 (1995).-   20. V. E. Henrich and P. A. Cox, The Surface Science of Metal    Oxides, Cambridge University Press, Cambridge (1994).-   21. Noguera, Physics and Chemistry of Oxide Surfaces, Cambridge    University Press, Cambridge (1996).-   22. G. Lu, A. Linsebigler, and J. T. Yates Jr., J. Chem. Phys.    102, p. 4657 (1995).-   23. N. Rusu and J. T. Yates Jr., Langmuir 13, p. 4311 (1997).-   24. L. Xio-e, A. N. M. Green, S. A. Haque, A. Mills, J. R.    Durrant, J. Photochem. Photobiol. A 162, p. 253 (2004).-   25. M. Peiro, G. Doyle, A. Mills, and J. R. Durrant, Adv. Mater.    17, p. 2365 (2005).-   26. Thiel and T. E. Madley, Surf. Sci. Rep. 7, p. 211 (1987).-   27. M. A. Henderson, Surf. Sci. Rep. 46, p. 1 (2002).-   28. U. Diebold, Surface Science Reports 48, p. 53 (2003).-   29. O. Legrini, E. Oliveros, and A. M. Braun, Chem. Rev. 93, p. 671    (1993).-   30. Heller, Acc. Chem. Res. 28, p. 503 (1995).-   31. L. Perkins and M. A. Henderson, J. Phys. Chem. B 105, p. 3856    (2001).-   32. U. Diebold, Surface Science Reports 48, p. 53 (2003).-   33. Wilson, Chemical & Engineering News 1, p. 29 (1996).-   34. U. Diebold, Surface Science Reports 48, p. 53 (2003).-   35. L. Xio-e, A. N. M. Green, S. A. Haque, A. Mills, J. R.    Durrant, J. Photochem. Photobiol. A 162, p. 253 (2004).-   36. M. Peiro, G. Doyle, A. Mills, and J. R. Durrant, Adv. Mater.    17, p. 2365 (2005).-   37. T. A. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds, Handbook    of Conducting Polymers 2^(nd) ed., Eds, Dekker, New York (1998).-   38. G. Hadziioannou and P. F. van Hutten, Semiconducting Polymers,    Eds, Wiley-VCH, Weinheim (2000).-   39. J. Campbell, D. D. C. Bradley, and D. G. Lidzey, J. Appl. Phys.    82, p .6326 (1997).-   40. H. -F. Meng and Y. -S. Chen, Phys. Rev. B 70, p. 115208 (2004).-   41. D. Parker, J. Appl. Phys. 75, p. 1656 (1994).-   42. O'Brien, M. S. Weaver, D. G. Lidzey, and D. C. Bradley, Appl.    Phys. Lett. 69, p. 881 (1996).-   43. L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett.    70, p. 152 (1997).-   44. W. Ma, P. K. lyer, X. Gong, B. Liu, D. Moses, G. C. Bazan,    and A. J. Heeger, Adv. Mater. 17, p. 274 (2005).-   45. H. Becker, S. E. Bums, and R. H. Friend, Phys. Rev. B 56, p.    1893 (1997).-   46. S. H. Kim, J. Y. Kim, S. H. Park, and K. Lee, Proc. SPIE Vol.    5937, p. 59371G1 (2005).-   47. L. A. Pettersson, L. S. Roman, and O. Inganäs, J. Appl. Phys.    86, p. 487 (1999).-   48. T. Stübinger and W. Brütting, J. Appl. Phys. 90, p. 3632 (2001).-   49. H. Hänsel, H. Zettl, G. Krausch, R. Kisselev, M. Thelakkat, and    H.-W. Schmidt, Adv. Mater. 15, p. 2056 (2003).-   50. H. J. Snaith, N. C. Greenham, and R. H. Friend, Adv. Mater.    16, p. 1640 (2004).-   51. Meizer, E. J. Koop, V. D. Mihaletchi, and P. W. M. Blom, Adv.    Funct. Mater. 14, p. 865 (2004).-   52. O'Regan and M. Gräzel, Nature 353, p. 737 (1991).-   53. U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissörtel, J.    Salbeck, H. Spreitzer, and M. Grätzel, Nature 395, p. 583 (1998).-   54. C. Arango, L. R. Johnson, V. N. Bliznyuk, Z. Schlesinger, S. A.    Carter, and H.-H. Hörhold, Adv. Mater. 12, p. 1689 (2000).-   55. J. Breeze, Z. Schlesinger, S. A. Carter, and P. J. Brock, Phys.    Rev. B 64, p. 125205 (2001).-   56. M. Thelakkat, C. Schmitz, and H.-W. Schmidt, Adv. Mater. 14, p.    577 (2002).-   57. Wang, J. Swensen, D. Moses, A. J. Heeger, J. Appl. Phys. 93, p.    6137 (2003).-   58. T. Sugimooto, et al., J. Colloid Interface Sci. 259, 43-52    (2003).-   59. W. Shangguan, et al., Sol. Energy Mater. Sol. Cells 80, 433-441    (2003).-   60. S. Lee, et al., Chem. Mater. 16, 4292-4295 (2004).-   61. Z. Zhong, et al., Chem. Mater. 17, 6814-6818 (2005).-   62. U. Scherf and E. J. W. List, Adv. Mater. 14, p.477 (2002).-   63. Spreitzer, H. Becker, E. Kluge, W. Kreuder, H. Schenk, R.    Demandt, and H. Schoo, Adv. Mater. 10, p. 1340 (1998).-   64. S. H. Kim, J. Y Kim, S. H. Park, and K. Lee, Proc. SPIE Vol.    5937, p. 59371G1 (2005)-   65. S. H. Kim, J. Y Kim, S. H. Park, and K. Lee, Appl. Phys. Lett.,    (2005).-   66. J. H. Park, O. O. Park, J.-W. Yu, J. K. Kim, and Y. C. Kim,    Appl. Phys. Lett. 84, p. 1783 (2004).-   67. S. H. Kim, J. Y Kim, S. H. Park, and K. Lee, Proc. SPIE Vol.    5937, p. 59371G1, (2005).-   68. S. H. Kim, J. Y Kim, S. H. Park, and K. Lee, Appl. Phys. Lett.,    (2005).-   69. T. D. Anthopoulos, D. M. de Leeuw, E. Cantatore, S.    Setayesh, E. J. Meijer, C. Tanase, J. C. Hummelen, and P. W. M.    Blom, Appl. Phys. Lett. 85, p. 4205 (2004).-   70. T. D. Anthopoulos, C. Tanase, S. Setayesh, E. J. Meijer, J. C.    Hummelen, P. W. M. Blom, and D. M. de Leeuw, Adv. Mater. 16, p. 2174    (2004).-   71. Tapponnier, I. Biaggio, and P. Gruner, Appl. Phys. Lett. 86, p.    112114 (2005).

SUMMARY OF THE INVENTION

A photovoltaic cell is provided comprising a first electrode, a secondelectrode, a photoactive, charge-separating layer comprising asemiconducting polymer blended with a suitable acceptor between thefirst and the second electrodes, and a passivating layer adapted toenhance the lifetime of the photovoltaic cell. The passivating layercomprises a substantially amorphous titanium oxide having the formula ofTiO_(x) where x represents a number from 1 to 1.96.

In one aspect, a method of preparing a photovoltaic cell comprising aphotoactive polymer layer is provided. The method comprises the step ofapplying a solution of a titanium oxide precursor to form a layer ofsubstantially amorphous titanium oxide having the formula of TiO_(x)where x represents a number from 1 to 1.96.

BRIEF DESCRIPTION OF THE DRAWINGS

These and various other features and advantages of the present inventionwill become better understood upon reading of the following detaileddescription in conjunction with the accompanying drawings and theappended claims provided below, where:

FIG. 1 is a schematic illustrating a polymer light-emitting diode (PLED)structure comprising a TiO_(x) layer in accordance with one embodimentof the invention;

FIG. 2 is a schematic illustrating a polymer solar cell comprising aTiO_(x) layer in accordance with one embodiment of the invention;

FIG. 3 is a schematic illustrating a n-type field-effect transistor(FET) structure comprising a TiO_(x) layer in accordance with oneembodiment of the invention;

FIG. 4 is a diagram illustrating energy levels for a device having anITO/PEDOT:PSS/MEH-PPV/TiO_(x)/Al structure in accordance with oneembodiment of the invention;

FIG. 5A is an atomic force microscope (AFM) scan of the surface of aTiO_(x) layer in accordance with one embodiment of the invention;

FIG. 5B is an X-ray diffraction pattern of a TiO_(x) layer and itscrystalline form after conversion at 500° C. in accordance with oneembodiment of the invention;

FIG. 5C is a graph showing an absorption spectrum of a TiO_(x) film inaccordance with one embodiment of the invention. The spectrum shows thatthe TiO_(x) film is substantially transparent in the visible range;

FIG. 6A is photoluminescence (PL) spectra of polyfluorene (PF) filmswith and without a TiO_(x) layer before annealing in accordance with oneembodiment of the invention;

FIG. 6B is PL spectra of PF films with and without a TiO_(x) layer afterannealing for 15 hours at 150° C. in the air in accordance with oneembodiment of the invention;

FIG. 7 is an X-ray photoelectron spectroscopy (XPS) of O_(1s) in thepolymer in structures of glass/polymer and glass/polymer/TiO_(x) inaccordance with one embodiment of the invention;

FIG. 8A is a graph showing current density-voltage (J-V) characteristicsfor polymer light-emitting devices comprising MEH-PPV polymer with andwithout a TiO_(x) layer in accordance with one embodiment of theinvention;

FIG. 8B is a graph showing brightness-voltage (L-V) characteristics forpolymer light-emitting devices comprising MEH-PPV polymer with andwithout a TiO_(x) layer in accordance with one embodiment of theinvention;

FIG. 9 is a graph comparing the luminous efficiency of PLEDs with andwithout a TiO_(x) layer in accordance with one embodiment of theinvention;

FIG. 10 is a schematic illustrating the charge injection for PLEDs withand without an electron injection/transport layer in accordance with oneembodiment of the invention;

FIG. 11A is a graph illustrating device characteristics of PLEDs that donot include a TiO_(x) layer;

FIG. 11B is a graph illustrating device characteristics of PLEDs thatinclude a TiO_(x) layer in accordance with one embodiment of theinvention;

FIG. 12 is a graph comparing the brightness and luminous efficiency as afunction of storage time for PLEDs with and without a TiO_(x) layer inaccordance with one embodiment of the invention;

FIG. 13A is a graph showing current density-voltage (J-V)characteristics of polymer solar cells that do not include a TIO_(x)layer;

FIG. 13B is a graph showing current density-voltage (J-V)characteristics of polymer solar cells that include a TIO_(x) layer inaccordance with one embodiment of the invention;

FIG. 14 is a graph comparing the power conversion efficiency as afunction of time for polymer solar cells with and without a TiO_(x)layer in accordance with one embodiment of the invention;

FIG. 15 is a graph comparing transfer characteristics of PCBM FETs withand without a TiO_(x) capping layer in accordance with one embodiment ofthe invention; the typical n-type I_(ds) versus V_(ds) characteristicsof a PCBM-FET with a TiO_(x) capping layer are shown in an inset in FIG.15;

FIG. 16A is a graph showing changes of transfer characteristics of PCBMFETs that do not include a TiO_(x) capping layer in accordance with oneembodiment of the invention;

FIG. 16B is a graph showing changes of transfer characteristics of PCBMFETs that include a TiO_(x) capping layer;

FIG. 17 is a graph showing the field-effect mobility of PCBM FETs withand without a TiO_(x) capping layer versus exposure time to the air inaccordance with one embodiment of the invention;

FIG. 18 is a graph showing the field-effect mobility of P3HT FETs withand without a TiO_(x) capping layer versus exposure time to the air inaccordance with one embodiment of the invention;

FIG. 19A is a schematic illustrating the spatial distribution of thesquared optical electric field strength |E|² inside the devices having astructure of ITO/PEDOT/Active-Layer/Al (left) and a structure ofITO/PEDOT/Active-Layer/Optical Spacer/Al (right);

FIG. 19B is a schematic illustrating a device structure with a briefflow chart of the steps involved in preparation of a TiO_(x) layer inaccordance with one embodiment of the invention;

FIG. 19C is a schematic showing the energy level of the singlecomponents of the photovoltaic cell shown in FIG. 19B;

FIG. 20A is a graph showing incident monochromatic photon to currentcollection efficiency (IPCE) spectra for devices with and without aTiO_(x) optical spacer layer;

FIG. 20B is a graph showing the change in absorption spectrum resultingfrom addition of an optical spacer. The lower dashed line represents theabsorption of P3HT:PCBM obtained from transmittance measurements. Theinset is a schematic description of the optical beam path in thesamples;

FIG. 21A is a graph showing current density-voltage (J-V)characteristics of polymer solar cells with and without a TiO_(x)optical spacer illuminated with 25 mW/cm² at 532 nm;

FIG. 21B is a graph showing current density-voltage (J-V)characteristics of polymer solar cells with and without a TiO_(x)optical spacer under AM1.5 illumination from a calibrated solarsimulator with an intensity of 90 mW/cm²; and

FIG. 22 is a schematic illustrating the mechanism for enhancing lifetimeof the devices comprising a TiO_(x) layer in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Various embodiments of the invention are described hereinafter withreference to the figures. It should be noted that some figures areschematic and the figures are only intended to facilitate thedescription of specific embodiments of the invention. They are notintended as an exhaustive description of the invention or as alimitation on the scope of the invention. In addition, one aspectdescribed in conjunction with a particular embodiment of the presentinvention is not necessarily limited to that embodiment and can bepracticed in any other embodiments of the present invention. Forinstance, various embodiments are provided in the drawings and thedescription in connection with polymer light-emitting diodes,photovoltaic cells, and field-effect transistors. It will be appreciatedthat the claimed invention may also be used in other electronic devices.

In general, the invention provides a structure useful in variouselectronic devices. The structure comprises a polymer layer having afirst surface and a second surface, and a substantially amorphousTiO_(x) layer on the first surface, where in the formula of TiO_(x), xrepresents a number from 1 to 1.96, preferably from 1.1 to 1.9, and morepreferably from 1.2 to 1.9. These values represent from 50% to 98% fulloxidation, preferably 55% to 95%, and more preferably 60% to 95% fulloxidation.

In some embodiments, the invention provides a structure comprising apolymer layer having two opposing sides and a substantially amorphousTiO_(x) layer on each of the opposing sides, wherein in the formula ofTiO_(x), x represents a number from 1 to 1.96, preferably from 1.1 to1.9, and more preferably from 1.2 to 1.9.

The polymer layer in the structures of the invention can be formed ofvarious polymers that are active or functional in various electronicdevices. Active polymers suitable for the invention include conductingor semiconducting polymers, and luminescent polymers, known moregenerally as conjugated polymers with molecule structures well known inthe art. Various exemplary polymers are provided below in connectionwith specific applications.

The thickness of the amorphous TiO_(x) layer can range from 5 to 500 nm,depending on specific applications. In most applications, the thicknesscan range from 5 to 100 nm. In some applications, good results can beobtained with the thickness ranging from 10 to 50 nm, or from 10 to 40nm.

In some embodiments, the invention provides an electronic devicecomprising a first electrode, a second electrode, an active polymerlayer positioned between the first and the second electrode, and asubstantially amorphous TiO_(x) layer between the active polymer layerand the second electrode, wherein in the formula of TiO_(x), xrepresents a number from 1 to 1.96, preferably from 1.1 to 1.9, and morepreferably from 1.2 to 1.9. Exemplary electronic devices include but arenot limited to diodes, light-emitting diodes, photodiodes, field-effecttransistors, photodetectors, and photovoltaic cells etc.

Solution-Processed Titanium Oxide (TiO_(x)) Layer in Polymer Diodes,Photodiodes and Light-Emitting Diodes

FIG. 1 schematically shows a light-emitting diode (LED) structurecomprising a TiO_(x) layer in accordance with one embodiment of theinvention. As shown, the LED is a thin-film device fabricated in ametal-insulator-metal configuration. The LED comprises a substrate suchas glass, a high work function electrode such as transparent indium-tinoxide and a hole injection layer such as, for example, poly(3,4-ethylenedioxylenethiophene)-polystyrene sulfonic acid(ITO/PEDOT:PSS) bilayer electrode deposited on the substrate, a low workfunction electrode such as metal aluminum of thickness around 100 nm,and a luminescent polymer layer sandwiched between the two electrodes.The high work function electrode injects hole carriers. The low workfunction electrode injects electron carriers. The low mobility of thecharge carriers in polymers (μ˜10⁻¹-10⁻⁶ cm²/Vs) typically requires thatthe thickness of the active layer be less than a few hundred nanometers.

A layer of TiO_(x) is formed on the luminescent polymer layer. Asdescribed in more detail below, a TiO_(x) layer can be formed by asolution-based sol-gel process, which is desirable for fabrication ofthe active polymer layer. The thickness of the TiO_(x) layer can rangefrom 5 to 500 nm. In one embodiment, a TiO_(x) layer having a thicknessof about 20 nm provides good device performance and lifetime for theLED. In the formula of TiO_(x), x represents a number less than 2 suchthat the material is a “suboxide.” In general, x in the formula ofTiO_(x) is a number from 1 to 1.96, preferably from 1.1 to 1.9, and morepreferably from 1.2 to 1.9. These values represent from 50% to 98% fulloxidation, preferably 55% to 95%, and more preferably 60% to 95% fulloxidation.

By introducing a TiO_(x) layer between a luminescent polymer layer and ametal electrode, the LED performance is significantly enhanced. Theenhanced performance can be contributed to the specific properties ofthe new TiO_(x) materials summarized as follows:

-   -   Energy levels of the bottom of the conduction band (LUMO) and        the top of the valence band (HOMO) well-matched with the        electronic structure requirements (electron accepting and        electron transporting, but hole blocking);    -   Relatively high electron mobility (μ_(e)≈1.7×10⁻⁴ cm²/Vs) as        determined by time-of-flight measurements;    -   Sol-gel process compatible with solution processing of polymer        electronics;    -   Transparency in the visible range with an energy band gap around        3.7 eV; and    -   TiO_(x) layer formation on top of an active polymer without        disturbing the polymer layer(s) below.

To achieve efficient electroluminescence (EL), a balanced bipolarinjection and transport of carriers is needed. Improved electroninjection can be achieved by choosing a low work function metal as thecathode material. Higher efficiencies can be achieved by confiningelectrons and holes within the emitting layer by using multilayer devicestructures with hole transport (electron blocking) layer on the cathodeside and an electron transport (hole blocking) layer on the anode side.The TiO_(x) layer inserted between the cathode and the emitting layeraccording to embodiments of the invention can effectively function as anelectron transport and a hole blocking layer, and as a result, enhancethe device performance.

There are other beneficial effects by inserting a TiO_(x) layeraccording to embodiments of the invention: preventing diffusion of metalions from the cathode into the luminescent polymer layer and quenchingof luminescence by proximity to the metal cathode. Diffusion of metalions into the polymer layer may reduce the lifetime of the device.Because of diffusion, alkali metals are typically not used as cathodematerials as the devices may quickly short out, although this problem isless severe for divalent alkaline earth metals. The device lifetime issignificantly longer with Ba as the cathode material than with Ca (thehigher mass of Ba inhibits diffusion). The diffusion problem can beeliminated or significantly reduced by inserting a TiO_(x) layeraccording to embodiments of the invention.

When the average distance between the cathode and the emittingoscillators within the luminescent polymer is too small, the losses fromthe metallic electrode quench the luminescence. This quenching effect isparticularly harmful in devices in which the electron mobility issmaller than the hole mobility (typically the case in semiconductingpolymers) since the recombination zone is closer to the cathodeinterface. This quenching problem can be largely eliminated by insertinga TiO_(x) layer between the luminescent polymer and the metal cathode.

The lifetime of the light-emitting diodes can be extended by inserting aTiO_(x) layer between the polymer emitting layer and the metal cathode.This benefit will be demonstrated in more detail in the Examplesprovided below.

The TiO_(x) films according to embodiments of the invention can beprepared using a sol-gel processed TiO_(x) precursor solution as will bedescribed in more detail below. Atomic force microscope (AFM) scans showthat the resulting TiO_(x) films are smooth with surface featuressmaller than a few nanometers and is substantially amorphous. TheTiO_(x) forms a high quality film on top of the active polymer layer.

The energy levels of the bottom of the conduction band (LUMO) and thetop of the valence band (HOMO) of the TiO_(x) material obtained fromoptical absorption and Cyclic Voltammetry (CV) data are shown in FIG. 4.The HOMO and LUMO energy levels for the other materials in FIG. 4 areknown in the art. The energy level diagram shown in FIG. 4 demonstratesthat the TiO_(x) layer satisfies the electronic structure requirementsof an electron transport layer: the conduction band edge of TiO_(x) is4.4 eV, which is well matched with the energy level of Al cathode (4.3eV). Because of the large band gap of TiO_(x), holes are blocked at thepolymer-TiO_(x) interface.

Solution-Processed Titanium Oxide (TiO_(x)) as an Optical Spacer andElectron Transport Layer in Polymer Solar Cells and Photodetectors

FIG. 2 schematically shows a polymer-based photovoltaic cell orphotodetector comprising a TiO_(x) layer in accordance with oneembodiment of the invention (a photovoltaic cell operates in reversebias functions as to a photodetector). The photovoltaic cell orphotodetector is a thin film device and fabricated in ametal-insulator-metal configuration. As shown, the device comprises asubstrate such as glass, a transparent high work electrode formed on thesubstrate for collecting hole carries such as a bilayer electrodecomprising a hole injecting layer such as, for example,poly(3,4-ethylenedioxylenethiophene)-polystyrene sulfonic acid(PEDOT:PSS) and indium-tin-oxide (ITO), a low work function metalelectrode such as aluminum (or Calcium or Barium, for example) forcollecting electron carriers, and an absorbing and charge separatingbulk heterojunction layer with a thickness of approximately 100 nmsandwiched between the two charge selective electrodes. Other materialssuch as conducting oxides, metallic polymers and the like well known inthe art can also be used for the transparent electrode. The workfunction difference between the two electrodes provides a built-inpotential that breaks the symmetry, thereby providing a driving forcefor the photo-generated electrons and holes toward their respectiveelectrodes. By way of example, the bulk heterojunction layer can bepoly(3-hexylthiophene) and [6,6,]-phenyl-C₆₁-butyric acid methyl ester(P3HT:PCBM).

A titanium oxide (TiO_(x)) layer can be deposited on top of the activepolymer layer using a solution-based sol-gel process as will bedescribed in more detail below. In the formula of TiO_(x), x representsa number of less than 2 such that the material is a “suboxide.” Usually,x is a number from 1 to 1.96, preferably from 1.1 to 1.90, and morepreferably from 1.2 to 1.90. The TiO_(x) layer significantly improvesthe power conversion efficiencies and device lifetime.

Introducing a TiO_(x) layer as an optical spacer between an active layerand a metal electrode in a photovoltaic cell changes the spatialredistribution of light intensity inside the device. TiO_(x) is an idealmaterial for an optical spacer because it is a good acceptor and anelectron transport material with a conduction band edge lower in energythan that of the lowest unoccupied molecular orbital (LUMO) of C₆₀, andthe LUMO is close to the Fermi energy of the collecting metal electrode.TiO_(x) is transparent to light with wavelengths within the solarspectrum.

A TiO_(x) layer improves the performance of polymer photovoltaic cells.The power conversion efficiencies of the devices can be increased byapproximately 50% compared to similar devices fabricated without aTiO_(x) optical spacer. A TiO_(x) layer also improves the lifetime ofpolymer photovoltaic cells as shown in the following Examples.

Solution-Processed Titanium Oxide (TiO_(x)) as a Capping Layer inPolymer Field Effect Transistors and Other Plastic Electronic Devices

FIG. 3 schematically shows a field-effect transistor (FET) structurecomprising a TiO_(x) layer in accordance with one embodiment of theinvention. As shown, the FET structure comprises a substrate such as aheavily doped n-type Si wafer. The doped n-type Si wafer functions as agate electrode. Other substrates such as for example glass, flexibleplastic substrates or free standing metal foils coated with aninsulating layer can also be used. A SiO₂ layer (gate dielectric) with athickness of such as 200 nm is thermally grown on the substrate. Thegate dielectric layer can also be made from a wide variety of otherinsulators. The source and drain electrodes (e.g. Al, Au, Ag, etc.) canbe deposited on the dielectric layer by methods well known in the artsuch as by e-beam evaporation or metal vapor deposition after patterningusing shadow masks or standard photolithographic methods. Asemiconducting polymer layer such as P3HT or an organic semiconductinglayer such as PCBM is deposited on the gate dielectric layer and coversthe source and drain electrodes. The FET channel is defined by thesource and drain electrodes. A TiO_(x) layer is formed on thesemiconducting polymer layer using solution processing method as will bedescribed in more detail below. It should be noted that FIG. 3 shows abottom contact configuration in which metal source and drain electrodesare deposited on the dielectric layer. Alternatively, the source anddrain electrodes can be deposited on the top of the semiconductingpolymer layer. In either case, the field induced carriers are confinedwithin the semiconducting layer to a thickness of a few nanometers nearthe interface with the gate dielectric.

As will be demonstrated in more detail in the following Examples, a FETcomprising a TiO_(x) layer significantly improves the device performanceand lifetime. While the invention is not limited to any theories, it isbelieved that a TiO_(x) layer acts as a barrier layer and a scavenginglayer that prevents the diffusion of oxygen and humidity into the activepolymer layer, thereby increasing the device lifetime by factorsapproaching two orders of magnitude. Moreover, the solution-based lowtemperature process for depositing a TiO_(x) layer is compatible withthe device architectures for FETs fabricated from semiconductingpolymers. The TiO_(x) layer reduces the sensitivity to oxygen and watervapor to a point where simple barrier materials might be sufficient toenable the lifetime required for printed, flexible, plastic electronics.

It should be pointed out that TiO_(x) layers can be positioned betweenthe active organic layer and one or both of the electrodes. In addition,the advantages of a TiO_(x) layer can be realized when it is applied asan overlayer or outer boundary layer in polymer-based electronicdevices. Thus, one can advantageously employ one, two or even threeTiO_(x) layers in these devices.

Solution Processing

The TiO_(x) layer according to embodiments of the invention can beincorporated into multilayer microelectronic or micro optoelectronicdevices. Such devices may include one or more organic polymer layers.These organic polymer layers can provide a substrate for the devices orin many embodiments, are present as conducting, semiconducting, or otherfunctional active layers. The processing conditions for applying TiO_(x)layers need to be compatible with the polymer layers which are moresensitive to high temperatures than the metal layers, inorganicsemiconducting layers, silicon layers and glass layers that are oftenfound in microelectronic devices. In addition, organic polymer layersare more sensitive to certain types of solvents than many of theinorganic materials described above.

Accordingly, while any compatible processing method may be used to applyTiO_(x) layers, solvent processing is preferred. In solvent processing,a layer of a solution or suspension such as a colloidal suspension ofone or more TiO_(x) precursors is applied. Solvent is removed, mostcommonly by evaporation to yield a continuous thin layer of TiO_(x), ora TiO_(x) precursor which is converted to TiO_(x) upon furtherprocessing such as mild heating. While the invention is not limited toany theories, it is believed that the precursor converts to TiO_(x) byhydrolysis and condensation processes as follows:Ti(OR)₄+4H₂O>TiO_(x)+YROH.

The TiO_(x) precursor can be a titanium alkoxide such as titanium(IV)butoxide, titanium(IV) chloride, titanium(IV) ethoxide, titanium(IV)methoxide, titanium(IV) propoxide. Other titanium sources such asTi(SO₄)₂ and so on can also be used. Such materials are commonlyavailable and soluble in lower alkanols such as C₁-C₄ alkanols which aregenerally compatible with and nondestructive to other organic polymerlayers commonly found in microelectronic devices. Alkoxyalkanols such asmethoxy-ethanol and the like can also be used. The solvents selectedshould not react with the TiO_(x) precursor. Therefore, care should betaken when aqueous solvents or mixed aqueous/organic solvents are usedduring processing as the water component can cause premature reactionsuch as hydrolysis of the TiO_(x) precursor. Another factor to beconsidered in selecting a titanium source and solvent is the ability ofthe precursor solution to wet the substrate upon which the solution isto be spread. The lower alkanol-based solutions/suspensions describedabove provide good wetting with organic layers.

The titanium concentration in the solution/suspension can vary from aslow as 0.01% by weight to as high as 10% by weight, or greater. In someembodiments, titanium concentration ranging from about 0.5 to 5% byweight has given good results.

The TiO_(x) precursor solution/suspension can be spread using variousconventional methods. In some embodiments, spin casting is used and hasprovided good results.

The TiO_(x) layer is formed by heating the solution of startingmaterials for a time and at a temperature suitable to react the startingmaterials but not so high as to cause conversion of the startingmaterials to a full stoichiometric oxide. Temperatures of from about 50degrees centigrade to about 150 degrees centigrade and times of fromabout 0.1 hour (at higher temperatures) to about 12 hours (at lowertemperatures) can be employed. In some embodiments, the temperature canrange from about 80 degrees centigrade to about 120 degrees centigradefor a time period from 1 to 4 hours, with the higher temperatures usingthe shorter times and the lower temperatures needing the longer times.

It is desirable to exclude oxygen during the casting and heating of thesolution of TiO_(x) precursors. This prevents premature conversion ofthe precursor to TiO_(x) or conversion of the TiO_(x) precursor to TiO₂full oxide. This can be accomplished by carrying out the casting andsolution preparation under vacuum or in an inert atmosphere such asargon or nitrogen atmosphere.

This invention will be further described with reference to the followingExamples. The Examples are provided to illustrate the invention and arenot intended to limit the scope of the invention in any way.

EXAMPLE 1 Solution-Processed Titanium Oxides

The TiO_(x) material was prepared using a novel sol-gel procedure asfollows: 10 mL titanium(IV) isopropoxide (Ti[OCH(CH₃)₂]₄, 99.999%,Sigma-Aldrich Corporation) was mixed with 50 mL 2-methoxyethanol(CH₃OCH₂CH₂ 0H, 99.9+%, Sigma-Aldrich) and 5 mL ethanolamine(H₂NCH₂CH₂OH, 99+%, Sigma-Aldrich) in a three-necked flask equipped witha condenser, thermometer, and an argon gas inlet/outlet respectively.The mixed solution was then heated to 80° C. for 2 hours in a siliconoil bath under magnetic stirring, followed by heating to 120° C. for 1hour. The two-step heating (at 80° C. and 120° C.) was then repeated. ATiO_(x) precursor solution was prepared in isopropyl alcohol.

Dense TiO_(x) layers were prepared from the TiO_(x) precursor solution.The precursor solution was spin-cast in the air on top of asemiconducting polymer layer comprising P3HT with thicknesses rangingfrom 20 to 40 nm. Subsequently, the films were heated at 80° C. for 10minutes in the air. During the process the precursor converted to asolid-sate TiO_(x) layer.

FIG. 5A is an atomic force microscope scan showing that the resultingTiO_(x) films were substantially smooth and transparent with surfacefeatures smaller than a few nm. Analysis by X-ray PhotoelectronSpectroscopy (XPS) revealed an oxygen deficiency at the surface of thethin film samples with Ti:O ratio of 42.1:56.4 (% ratio); hence titanium“suboxide,” or TiO_(x). was formed.

X-ray diffraction (XRD) results shown in FIG. 5B confirm that theTiO_(x) film is substantially amorphous. The physical properties of thefilms are excellent. Time of flight measurements on these TiO_(x) filmsindicate that the electron mobility (μ_(e)) is μ_(e)≈1.7×10⁻⁴ cm²/Vs,somewhat higher than the mobility values obtained from amorphous oxidefilms prepared by typical sol-gel processes. The absorption spectrum ofthe film exhibits a well-defined absorption edge at E_(g)≈3.7 eV asshown in FIG. 5C. Using optical absorption and Cyclic Voltammetry (CV)data, the energies of the bottom of the conduction band and the top ofthe valence band of the TiO_(x) material were determined as −4.4 eV and−8.1 eV, respectively, referenced to the vacuum. The TiO_(x) layersatisfies the electronic structure requirements of an inserting layer:the conduction band edge of TiO_(x) is −4.4 eV (relative to the vacuum),which is well matched with the Fermi level of the Al cathode (−4.3 eV);the valence band edge at −8.1 eV assures that the TiO_(x) functions as ahole blocking layer.

EXAMPLE 2 TiO_(x) as an Oxygen Barrier and an Oxygen Scavenging Layer

Comparison studies of photoluminescence (PL) stability of polyfluorene(PF) with and without a TiO_(x) layer were carried out to confirm theoxygen barrier and scavenging properties of the TiO_(x) layer. Fourfilms with the following structures were prepared by spin-casting:glass/PF, glass/TiO_(x)/PF, glass/PF/TiO_(x), andglass/TiO_(x)/PF/TiO_(x). The films Were then heated for 15 hours at150° C. in the air.

It is known that the PF type materials degrade with an appearance of along-wavelength emission around 500-600 nm after heating in the air.This green emission peak arises by energy transfer from singlet excitonson the PF chains to keto-defect sites that form by reaction with oxygenpresent in the luminescent polymer. Therefore, it is expected that thefour different samples would exhibit different peak intensities for thelong wavelength emission because of the shielding and oxygen scavengingeffect of the TiO_(x) layer.

FIG. 6A shows the initial PL spectra of all the films which are typicalof PF without any peak in the region of 500-600 nm. The initial PL colorwas pure blue. After the films were heated for 15 hours at 150° C. inthe air, the PF film without a TiO_(x) layer developed a pronounced peakin the PL emission spectrum in the 500-600 nm region, as shown in FIG.6B, and the emission color changed from blue to green. For the PF filmscovered by a TiO_(x) layer (glass/PF/TiO_(x) andglass/TiO_(x)/PF/TiO_(x)), the PL peak in the 500-600 nm spectral rangeis significantly reduced (almost completely eliminated); the emissioncolor remains blue. Note that the TiO_(x) layer provided some benefiteven when it was beneath the PF (glass/TiO_(x)/PF): the green emissionpeak is smaller than that emitted from the glass/PF film. Since theglass substrates (few mm thick) are excellent shielding materials, theintroduction of a TiO_(x) layer between the glass and PF would not beexpected to provide any barrier to oxygen or water vapor. However, theintensity difference of the green peak between the glass/PF andglass/TiO_(x)/PF samples (also a small difference between theglass/PF/TiO_(x) and glass/TiO_(x)/PF/TiO_(x) films) shows that theTiO_(x) layers have an effect of oxygen scavenging as well as oxygenshielding.

More direct evidence of the oxygen shielding and oxygen scavengingeffects of the TiO_(x) layers comes from X-ray photoelectronspectroscopy (XPS) measurements. This method was employed to directlycompare the oxygen concentration inside the polymers with and without aTiO_(x) layer. The XPS analysis was performed using VG ScientificESCALAB 250 XPS spectrometer equipped with a monochromated Al K-alphaX-ray source (hv=1486.6 eV) at 15 kV. The analysis area wasapproximately 500 μm in diameter. Utilizing alkoxy-substituted 2-phenylPPVs as a luminescent material, glass/polymer and glass/polymer/TiO_(x)films were prepared and subsequently annealed for 48 hours at 150° C. inair to accelerate the oxidation of the polymer films. Then in order tocompare the oxygen ratio of the two polymers, the TiO_(x) layer wasremoved from the glass/polymer/TiO_(x) sample by using the XPS depthprofiling technique. The measured polymer layers of both samples wereetched with a depth of around 10 nm to remove any surface oxygen.

FIG. 7 shows the relative ratio of O_(1s)/C_(1s) inside the polymerswith and without a TiO_(x) layer. The polymer without a TiO_(x) layerhas a high intensity peak of O_(1s)/C_(1s) with an asymmetric feature,whereas this signal is hardly detectable in the polymer layer coveredwith a TiO_(x) layer. These data provide direct evidence of oxygenbarrier and scavenging effects of the TiO_(x) layers of the invention inthe polymer-based electronic devices.

EXAMPLE 3 Polymer Diodes and Polymer Light-Emitting Diodes with EnhancedPerformance as a Result of a Titanium Oxide (TiO_(x)) Layer

Polymer diodes and LEDs were fabricated in the sandwich structure:ITO/PEDOT:PSS/Polymer/TiO_(x)/Al. The semiconducting polymer used inthis example was MEH-PPV available from Organic Vision Inc. Thethickness of the MEH-PPV layer was approximately 100 nm. The TiO_(x)precursor solution (1 wt %) was spin-cast (6000 rpm) onto thesemiconducting polymer layer with a thickness around 20 nm, and heatedat 80° C. for 10 minutes in the air. During this process the precursorconverted to TiO_(x). Subsequently the devices were pumped down invacuum (<10⁻⁶ Torr), and then Al electrode with a thickness around 150nm was deposited. The deposited Al electrode area defined an active areaof the device as 16 mm². The current density-voltage-luminancecharacteristics were measured using a Keithley 236 source measurementunit along with a calibrated silicon photodiode inside a glove box.

FIG. 8A-8B show the current density versus voltage (J-V) and brightnessversus voltage (L-V) characteristics of the devices comprising a TiO_(x)layer with various thicknesses (MEH-PPV as a semiconducting polymer) inthe forward direction. For the devices without a TiO_(x) layer, theturn-on voltage for current injection was about 5V. When a TiO_(x) layerwas inserted between the polymer and Al cathode, a significant increasein current density (j) was observed compared with the current density ofa conventional device without a TiO_(x) layer at the same voltage. Forexample, for the conventional device, the current density was j≈500mA/cm² at 8 V, but increased to j≈1500 mA/cm² at the same voltage forthe devices with a TiO_(x) layer. Since the hole transport was blocked,the enhanced current density indicates that electron injection isimproved. It should be noted that the J-V curves are not sensitive tothe thickness of the TiO_(x) layer between 10-30 nm.

The L-V curves shown in FIG. 8B demonstrate significantly enhancedperformance for devices as a result of the insertion of a TiO_(x)electron transport layer (ETL). For devices with a TiO_(x) layer, thebrightness increased dramatically over that of the conventional devicewithout a TiO_(x) layer. The device performance was sensitive to thethickness of the TiO_(x) layer. The device comprising a TiO_(x) layerwith a thickness of 20 nm exhibited a higher brightness than the othertwo devices which had a thickness of 10 nm and 30 nm respectively. Asshown in FIG. 9, the luminous efficiency of the 20 nm-thickness deviceis almost one order of magnitude higher than that of the conventionaldevice.

It should be pointed out that because Al was used as the cathode, theefficiency of the device was low compared to that of devices made withCa or Ba as the cathode material. Because structures are provided todemonstrate improved lifetime of diodes and LEDs as a result of theinsertion of a TiO_(x) layer (see Examples below), Ca or Ba materialswere not used as the device performance was monitored in the air.Nevertheless, the data in FIGS. 8 and 9 demonstrate relatively goodelectron transport through the TiO_(x) layer and relatively goodelectron transport across the interface between the TiO_(x) and thesemiconducting polymer.

FIG. 10 shows the electronic structure of an LED with an electrontransport layer (ETL). The ETL creates a barrier at the interface of twopolymers that blocks the flow of holes. As shown in FIG. 10, a dipoledouble layer forms at the interface. If the dipole layer is sufficientlythin, electrons can tunnel through the barrier into the π*-band of thesemiconducting polymer. As a result, the electron and hole currentsbecome more balanced.

EXAMPLE 4 Polymer Diodes and Light-Emitting Diodes with EnhancedLifetime as a Result of a Titanium Oxide (TiO_(x)) Electron TransportLayer

Polymer LEDs comprising a TiO_(x) layer between an active layer and Alelectrode as shown in FIG. 1 were fabricated. For comparison,conventional polymer LEDs without a TiO_(x) layer were also fabricated.In these experiments, “super yellow” (SY) polymer, a soluble derivativeof poly(paraphenylene vinylene, available from Covion Co. was used asthe luminescent polymer. A layer of PEDOT:PSS (Bayton P VP Al 4083)available from Bayton was spin-cast onto ITO to form a bilayer anode. Asolution of SY (0.7 wt.-% in toluene) was spin-cast (2000 rpm) on top ofthe PEDOT:PSS layer, and baked at 80° C. for 30 minutes. The thicknessesof the SY layer was about 100 nm. Then, a TiO_(x) precursor solution (1wt %) was spin-cast (6000 rpm) onto the SY emitting layer with athickness about 20 nm, and heated at 80° C. for 10 minutes in the air.During this process the precursor converted to TiO_(x). Subsequently thedevices were pumped down in vacuum (<10⁻⁶ Torr), and then Al electrodeswith thickness about 150 nm were deposited. The deposited Al electrodearea defined an active area of the devices as 16 mm². The currentdensity-voltage-luminance characteristics were measured using a Keithley236 source measurement unit along with a calibrated silicon photodiodeinside a glove box.

After fabrication and initial characterization, the devices were storedin the ambient atmosphere to monitor the degradation of the devicesversus storage time. No packaging or encapsulation was used except for aTiO_(x) layer between the SY layer and the cathode.

FIGS. 11A and 11B show the current density versus voltage (J-V) and theluminance versus voltage (L-V) characteristics of the devices measuredafter various storage periods in the air. The devices without a TiO_(x)layer initially exhibited characteristics typical of polymer LEDs madewith SY and Al cathode, with an onset voltage of ˜8 V and luminance ofL≈400 cd/m² at 13 V (FIG. 11A). After storage in the air, however, thedevice performance rapidly degraded. After three hours (180 minutes),the luminance dropped below 100 cd/m² at 13 V, corresponding to onefourth of the initial value, and became almost negligible after 8 hours(480 minutes). The onset voltage also increased considerably as thestorage time increased.

In contrast, the devices with a TiO_(x) layer showed a more robustbehavior as illustrated in FIG. 11B. The luminescence of the devicesremained almost unchanged after three hours in the air with L≈700 cd/m²at 13 V, and slightly decreased to ˜600 cd/m² at 13 V after 8 hours (480minutes). After 22 hours (1320 minutes) the device retained a brightnessof ˜400 cd/m² at 15 V. Remarkably, without any additional packaging, athin TiO_(x) layer (e.g., ˜30 nm) slowed the degradation byapproximately two orders of magnitude.

In addition to the enhanced lifetime, the performance of the TiO_(x)devices was also improved compared with that of conventional devices. Asshown in FIG. 12, the brightness and efficiency actually increasedinitially. For example, the brightness at 13V increased fromapproximately 700 cd/m² to about 1000 cd/m² during the first two hours,whereas the initial value of the conventional devices was only L≈400cd/m² at 13V and decayed rapidly to almost negligible values within fewhours. Therefore, a TiO_(x) layer provides an attractive approach toreducing the sensitivity of polymer LEDs to oxygen and water vapor.

Because of the reduced sensitivity, simple barrier materials might besufficient to provide long lifetime to diodes, diodes arrays, polymerLEDs and arrays of polymer LEDs in display and lighting applications.

EXAMPLE 5 Polymer Solar Cells with Enhanced Lifetime as a Result of aTitanium Oxide (TiO_(x)) Optical Spacer Layer

Polymer solar cells comprising a TiO_(x) layer as shown in FIG. 2 werefabricated using poly(3-hexylthiophene) (P3HT) as the electron donor and[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the electronacceptor. The ITO-coated glass substrates were cleaned in an ultrasonicbath with a detergent, distilled water, acetone, and isopropyl alcoholand then dried overnight in an oven at about 100° C. Highly conductingPEDOT:PSS was spin-cast (5000 rpm) with a thickness about 40 nm fromaqueous solution after treatment with UV-ozone for 40 minutes. Thesubstrates were dried at 140° C. for 10 minutes in the air, and thentransferred to a nitrogen filled glove box for spin-casting theP3HT:PCBM layer. The chloroform solution comprised of P3HT (1 wt. %) orP3HT (0.8 wt. %) was then spin-cast at 1200 rpm on top of the PEDOT:PSSlayer. The thickness of the active layer was about 200 nm. Then, aTiO_(x) layer (about 30 nm) was spin-cast (4000 rpm) on top of theP3HT:PCBM composite from the precursor solution (1 wt. %), and heated at80° C. for 10 minutes in the air. Thermal annealing was carried out bydirectly putting the samples on the hot plate at 150° C. for 10 minutesin a nitrogen filled glove box. Subsequently the device was pumped downin vacuum (<10⁻⁶ Torr), and an Al electrode with a thickness of about150 nm was deposited. The area of the Al electrode defined the activearea of the device as 4.5 mm². Thermal annealing was carried out bydirectly placing the completed devices without a TiO_(x) layer on a hotplate at 150° C. in a glove box filled with nitrogen gas. Afterannealing, the devices were put on a metal plate and cooled to roomtemperature before the measurements were carried out.

For calibration of solar simulators, the mismatch of the spectrum (thesimulating spectrum) obtained from the Xenon lamp (150 W Oriel) and thesolar spectrum using an AM1.5 filter was carefully minimized. The lightintensity was calibrated using a standard silicon photovoltaic (PV)solar cell from the National Renewable Energy Laboratory (NREL).Measurements were carried out with the solar cells inside a glove box byusing a high quality optical fiber to guide the light from the solarsimulator (outside the glove box). Current density-voltage curves weremeasured with a Keithley 236 source measurement unit.

The TiO_(x) layer improved the lifetime of polymer-based solar cells.FIGS. 13A-13B show the current density vs. voltage (J-V) characteristicsof a photovoltaic cell with and without a TiO_(x) layer under AM 1.5illumination at irradiation intensity of 100 mW/cm². The conventionaldevice without a TiO_(x) layer showed a typical photovoltaic responsewith device performance comparable to that reported in previous studies;the short circuit current (I_(sc)) was I_(sc)=10.7 mA/cm², the opencircuit voltage (V_(oc)) was V_(oc)=0.62, and the fill factor (FF) wasFF=0.60. These values correspond to a power conversion efficiency(η_(e)=I_(sc)V_(oc)FF/P_(inc), where P_(inc) is the intensity ofincident light) of η_(e)=4.0%.

When these conventional devices were stored in the ambient air, adramatic decrease in I_(sc) was observed as the storage time increased,I_(sc) dropped to <15% of the initial value after 36 hours (2160minutes). Note, however, that the V_(oc) remained almost constant at0.62 V, indicating that the devices still function properly withoutcatastrophic failure. For the device with a TiO_(x) layer, the initialperformance was comparable to those of the conventional devices withouta TiO_(x) layer; I_(sc)=10.8 mA/cm², V_(oc)=0.62 V, FF=0.61, yieldingη_(e)=4.1%. Note, however, that the conventional devices were fabricatedby using postproduction heat-treatment at 150° C. to improve theefficiency, whereas the devices with a TiO_(x) layer were prepared bypreheat-treatment. As a result, the initial performance of the twodevices were almost identical. However, the devices with a TiO_(x) layerexhibited quite different behavior with increased storage time. Thedevices with a TiO_(x) layer showed a much longer lifetime; even after36 hours storage in the air, I_(sc) remained at almost 90% of itsinitial value.

The lifetime enhancement of the devices including a TiO_(x) layer isevident in FIG. 14. The efficiency of the conventional devices decreasedabruptly to half of the initial value within first 200 minutes, and thencontinued to drop below η_(e)=1% after storage in the air for 1000minutes. The devices with a TiO_(x) layers retained at 3% efficiencyafter 2000 minutes; even after 8000 minutes, η_(e)=2% (half the initialvalue). The reduced fill-factor dominated the degradation of the deviceswith a TiO_(x) layer, thus the degradation appeared to be mostly aresult of an increase in series resistance. Thus, the data clearlydemonstrate that a TiO_(x) layer enhanced the lifetime of polymerphotovoltaic cells. Compared with the conventional devices without aTiO_(x) layer, the unpackaged lifetime was enhanced by a factor of 40.By also functioning as an optical spacer, a TiO_(x) layer offers thepotential for increasing the efficiency as well as the device lifetime.Because of the reduced sensitivity to oxygen and water vapor, simplebarrier materials might be sufficient to provide sufficiently longlifetime for commercial implementation.

EXAMPLE 6 Polymer Field-Effect Transistors with Enhanced Lifetime as aResult of a Titanium Oxide (TiO_(x)) Capping Layer

Polymer FETs were fabricated in a bottom contact geometry as shown inFIG. 3. The FET structures were fabricated on a heavily doped n-type Siwafer (which functioned as the gate electrode) with a 200 nm thickthermally grown SiO₂ layer (gate dielectric). The channel length (L) andthe channel width (W) of the devices were 5 μm and 1000 μm,respectively. Aluminum source and drain electrodes (50 nm) weredeposited on a SiO₂ insulating layer by e-beam evaporation. PCBM (orP3HT) were used as the active semiconductor layer in the channel. Beforedepositing P3HT (or PCBM) active layer, aluminum electrodes were etchedwith standard aluminum etchant to remove aluminum oxide layer. Afterdepositing the PCBM (or P3HT) by spin-casting, a TiO_(x) layer with athickness about 30 nm was spin-cast on top of the FET device. TheTiO_(x) solution was spin-cast at 5000 rpm for 60 seconds on top of thesemiconducting polymer layer. In this example, the TiO_(x) layer servesto reduce the sensitivity of the FET to oxygen and water vapor.

Electrical characterization of the device was performed using a Keithleysemiconductor parametric analyzer (Keithley 4200) under N₂ atmosphere.In order to investigate the environmental stability of the FET devices,the devices were taken out of the glove box and left in the air. Thedevice performance was periodically monitored as a function of time.

A TiO_(x) layer enhanced the lifetime of polymer field-effecttransistors (FETs). FIG. 15 compares the transfer characteristics ofPCBM-FETs with and without a TiO_(x) layer, measured just afterfabrication without any exposure to the air. The drain-source current(I_(ds)) curves versus applied gate voltage (V_(gs)) were typical ofn-channel organic FETs; the device performance was comparable to thatconventional devices. Moreover, the presence of a TiO_(x) layer on topof the active layer did not influence the device performance whenmeasured in vacuum without exposure to the air. After exposure to theair, however, the two devices exhibited quite different behavior asshown in FIGS. 16A-16B. For the devices without a TiO_(x) layer, I_(ds)decreased rapidly and the turn-on voltage (V_(to)) shifted to highervalues with increased exposure time, whereas the devices with a TiO_(x)capping layer showed a slow decrease in I_(ds) and small shift ofV_(to). It is well known that both the shift of V_(to) to higher valuesand the decrease in I_(ds) originate from the diffusion of oxygen andwater vapor into PCBM polymer. The data in FIGS. 16A and 16B demonstratethat a TiO_(x) layer reduced the diffusion of oxygen and water vaporinto polymer-based FETs.

The effect of a TiO_(x) capping layer is more pronounced in the study ofthe electron mobility (μ). The mobilities were extracted form the slopeof (|I_(ds)|)^(1/2) vs. V_(gs) (not presented here) in the saturationregion using following equation:I _(ds)=(WC _(i)/2L)μ(V _(gs) −V _(T))²where V_(T) is the threshold voltage, and C_(i) is the capacitance perunit area of insulating layer (for 200 nm layer of SiO₂, C_(i)=17nF/cm²). FIG. 17 shows the results obtained for μ as a function ofexposure time. While the mobility of the devices without a TiO_(x) layerdecreased rapidly (almost two orders of magnitude decrease within first100 minutes), the devices with a TiO_(x) capping layer were much morestable during exposure to the air with less than one order of magnitudedecrease even after 1000 minutes of air exposure.

The lifetime enhancement provided by a TiO_(x) is not limited to PCBM asthe semiconducting layer in the channel, but appears to be general. Forexample, FETs using P3HT polymer capped with a TiO_(x) layer alsoexhibited enhanced device lifetimes as shown in FIG. 18. Therefore, as aresult of a TiO_(x) capping layer and the associated reduced sensitivityto oxygen and water vapor, simple barrier materials might be sufficientto enable the lifetime required for printed, flexible, plasticelectronics.

The use of a TiO_(x) capping layer can also be used to extend thelifetime of other plastic electronic devices such as diodes,photodetectors and more generally plastic electronic circuits. Whenemployed as a capping layer for diodes, photodetectors or plasticelectronic circuits, the TiO_(x) capping layer does not play an activerole in the device operation but serves to enhance the device lifetime.

An innovative approach to enhancing the performance and lifetime ofelectronic devices is described herein. A solution-based sol-gel processis provided to fabricate a titanium oxide (TiO_(x)) layer on top of theactive polymer layer(s) in thin-film devices. By introducing asolution-based titanium (TiO_(x)) layer between an active layer and ametal such as aluminum cathode as an electron transport layer (ETL) inpolymer diodes and polymer light-emitting diodes (PLEDs), both thedevice performance and lifetime are enhanced. Field-effect transistors(FETs), photodiodes and photodetectors fabricated from semiconductingpolymers exhibit a similar lifetime extension with the addition of aTiO_(x) layer on top of the semiconducting polymer. The success of thisapproach originates from the excellent physical properties of the newTiO_(x) material, the specific process that enables low-temperaturedeposition of TiO_(x) on top of the semiconducting polymer layer, andthe oxygen/water protection and scavenging effects of TiO_(x). Theaddition of a TiO_(x) on top of the semiconducting polymer layerimproves the lifetime of unpackaged devices by nearly two orders ofmagnitude and thereby significantly reduces the barrier requirements ofpackaging materials for plastic electronics.

1. A photovoltaic cell comprising a first electrode, a second electrode,a photoactive, charge-separating layer comprising a semiconductingpolymer between the first and the second electrodes, and a passivatinglayer adapted to enhance lifetime of the photovoltaic cell, wherein saidpassivating layer comprises a substantially amorphous titanium oxidehaving the formula of TiO_(x) where x represents a number from 1 to1.96.
 2. The photovoltaic cell of claim 1 wherein in the formula ofTiO_(x), x represents a number from 1.1 to 1.9.
 3. The photovoltaic cellof claim 1 wherein in the formula of TiO_(x), x represents a number from1.2 to 1.9.
 4. The photovoltaic cell of claim 1 wherein the titaniumoxide layer has a thickness ranging from 5 to 500 nanometers.
 5. Thephotovoltaic cell of claim 1 wherein the titanium oxide layer has athickness ranging from 5 to 100 nanometers.
 6. The photovoltaic cell ofclaim 1 wherein the titanium oxide layer has a thickness ranging from 10to 40 nanometers.
 7. The photovoltaic cell of claim 1 wherein thetitanium oxide layer is positioned adjacent to the photoactive,charge-separating layer.
 8. The photovoltaic cell of claim 1 wherein thetitanium oxide layer is positioned between the photoactive,charge-separating layer and one of the first and the second electrodes.9. The photovoltaic cell of claim 1 wherein the titanium oxide layer isa boundary layer of the photovoltaic cell.
 10. The photovoltaic cell ofclaim 1 wherein the first electrode comprises a transparent holeinjecting layer and indium-tin-oxide (ITO) bilayer electrode, the secondelectrode comprises a metal electrode, the photoactive polymer layercomprises a heterojunction layer comprised of poly(3-hexylthiophene) and[6,6,]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM), and thepassivating layer is positioned between the second electrode and thephotoactive polymer layer.
 11. A method of preparing a photovoltaic cellcomprising a first electrode, a second electrode, and a photoactive,charge-separating layer comprising a semiconducting polymer between thefirst and second electrodes, which comprises the step of applying asolution of a titanium oxide precursor to form a layer of substantiallyamorphous titanium oxide having the formula of TiO_(x) where xrepresents a number from 1 to 1.96.
 12. The method of claim 11 whereinthe solution of the titanium oxide precursor is applied by spin-casting.13. The method of claim 11 wherein the solution of the titanium oxideprecursor is applied onto the photoactive, charge-separating layer. 14.The method of claim 11 further comprising the step of heating theapplied solution at a temperature from about 50° C. to about 150° C. 15.The method of claim 14 wherein the step of heating is performed at atemperature from about 80° C. to about 120° C.
 16. The method of claim11 wherein the titanium oxide precursor comprises titanium(IV) butoxide,titanium(IV) chloride, titanium(IV) ethoxide, titanium(IV) methoxide,titanium(IV) propoxide, and Ti(SO₄)₂.